1. Field of the Invention
The present invention relates to a polarizing beam splitter for splitting an incident light beam into two light beams which are polarized in mutually orthogonal directions, and an optical pick-up head comprising a polarizing beam splitter for recording and/or reproducing information on and/or from a magneto-optical record medium.
2. Related Art Statement
There have been proposed various polarizing beam splitters such as Nicol's prism and Glan-Thompson's prism for dividing an incident light beam into two orthogonally polarized light beams. In these polarizing beam splitters, the two orthogonally polarized light beams are derived by means of a single refraction or a combination of a single refraction and a single reflection. Further, there has been proposed a Cotton prism in which an incident light beam is divided into two orthogonally polarized light beams by means of a single reflection.
FIG. 1 is a schematic view showing a known Cotton prism. A prism 100 is consisting of a triangular prism made of a birefringent crystal whose optic axis A is coincided with an optical axis of an incident light beam I. The prism 100 is arranged with respect to the incident light beam I such that the incident light beam transmitted through an incidence surface 102 of the prism is made incident upon a reflection surface 101 of the prism at an incident angle of 45 degrees. Therefore, an optical axis of the light beam reflected by the reflection surface 101 becomes perpendicular to the optic axis A of the prism. Upon the reflection by the reflection surface 101, the incident light beam I is divided into two light beams, i.e. ordinary light beam O.sub.1 and extraordinary light beam O.sub.2. The prism 100 has different indices of refraction for the ordinary light beam O.sub.1 and extraordinary light beam O.sub.2, so that these beams are reflected into different directions and enamate from an exit surface 103 of the prism 100.
In the Cotton prism 100, the incidence light beam I should have a polarized component which is in a plane of incidence (P polarized component) and a polarized component which is perpendicular to the plane of incident (S polarized component). It should be noted that the plane of incidence is defined as a plane which includes the optical axis of the incident light beam and a normal to the reflection surface 101. When the incident light beam I includes these P and S polarized components, the P and S components are divided into the extraordinary light beam O.sub.2 and ordinary light beam O.sub.1, respectively. Therefore, when the incident light beam contains only the P polarized component or S polarized component, the incident light beam could not be split into two light beams. This often limits the application of the Cotton prism.
In Japanese Patent Application Laid-open Publication Kokai Hei 5-203810, there has been proposed a polarizing beam splitter which can avoid the above mentioned drawback. FIGS. 2A and 2B illustrate a construction of this known polarizing beam splitter. The polarizing beams splitter comprises a composite prism 110 including an azimuth rotator 111 and a birefringent crystal prism 112. An optic axis A of the birefringent crystal prism 112 is set to be substantially perpendicular to an optical axis of an incident light beam I and to be substantially parallel with optical axes of exiting light beams O.sub.1 and O.sub.2. The azimuth rotator 111 serves to rotate a polarizing direction of the incident light beam I, i.e. P polarized light beam by such an angle that a polarizing direction of the incident light beam impinging upon the birefringent crystal prism 112 is inclined by, for instance 45 degrees with respect to the optic axis A of the birefringent crystal. Therefore, both P and S polarized light components are made incident upon a reflection surface 113 of the birefringent crystal prism 112. Therefore, upon reflection at the reflection surface 13, the incident light beam is divided into two orthogonally polarized light beams, i.e. ordinary light beam O.sub.1 and extraordinary light beam O.sub.2. In this manner, this known polarizing beam splitter can divide the P polarized light beam I into the orthogonally polarized light beams O.sub.1 and O.sub.2.
In the known polarizing beam splitter shown in FIGS. 2A and 2B, the beam splitter is formed by cementing the azimuth rotator 111 and birefringent crystal prims 112 to each other, so that a manufacturing cost is liable to be increased. Moreover, it is required to align the optic axes of the azimuth rotator 111 and birefringent crystal prims 112 with each other, so that an adjustment is very cumbersome.